Magnetic resonance imaging (MRI) is a state of the art imaging technology which allows cross-sectional viewing of objects like the human body with unprecedented tissue contrast. MRI is based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules. The basis of both NMR and MRI is the fact, that atomic nuclei with non-zero spin have a magnetic moment. In medical imaging, usually nuclei of hydrogen atoms are studied since they are present in the body in high concentrations like for example water. The nuclear spin of elementary particles can resonate at a resonance frequency, if a strong DC magnetic field is applied. This magnetic resonance (MR) frequency is determined by the magnetic field. In an MRI scanner, the magnetic field matches a selected resonance frequency only at one position in space. Only at this position the presence of these particles can be detected. By varying this position step-by-step, an image can be measured. In practice, more sophisticated algorithms are used to achieve the image in a reasonable time from e.g. ‘slices’ of the investigated volume. Typical resonance frequencies for protons are in the range from 42 MHz to 300 MHz, corresponding to magnetic flux levels in the range of 1 T to 7 T.
The needed strong DC magnetic field (B0 field) is typically generated by superconducting magnets. In order to vary these fields, such that it matches a given radio frequency only at one position, a field gradient is generated using gradient coils. The field gradient can vary over time to achieve a scan. The frequency range in the gradient coils is low and reaches up to a maximum of 10 kHz.
To excite nuclear resonances, the RF coil generates a high frequency magnetic field at the nuclear resonance. To measure nuclear resonances, ‘sensor’ or ‘receiver’ coils are placed close to the region of interest, e.g. on the patient. These coils must be oriented such that their axis points are approximately in radial direction with respect to the axis of the MRI scanner. Often, a number of sensor coils are connected to a complete module, e.g. such a module may consist of 4×4 individual sensor coils. The module also includes additional electronics to process the measured signals.
Instead of using separate transmission and receiver coils it is also possible using combined receiver-transmitter coils. For example, only one coil arrangement can be used which can be switched between transmission and reception mode.
It is a goal of MR machine manufacturers to provide MR systems which ensure a reproducible system and high quality of MR images. High quality of MR images includes high image resolution at high image homogeneity. In order to test the performance of a magnetic resonance imaging system, typically MRI phantoms are used. A phantom is an artificial object of known size and composition that is imaged to test, adjust or monitor an MRI system's homogeneity, imaging performance and orientation aspects. A phantom may for example be a container of certain size and shape filled with a material which simulates the presence of human tissue. Phantoms can be generally further characterized with respect to human tissue similarity by their dielectric constant and electrical conductivity. In general, the electrical conductivity is directly related to the electrical load of RF coils. Further, the load upon an RF coil is directly related to a quality factor (Q) and impedance of RF coils. It has to be noted here, that coil loading is generally understood here as the interaction of any object with the RF coil which causes shifts of the resonance frequency and damping of the coil's resonance and hence reduction of the quality factor because of magnetic induction and dielectric losses in the object.
When using phantoms with a dielectric constant corresponding to the dielectric constant of tissue, the problem arises that such a high dielectric constant of tissue reduces the RF wavelength within the phantom resulting in the generation of standing waves in the phantom which greatly perturb the effective high frequency magnetic field (B1) at the nuclear resonance. Due to the electrical conductivity of the tissue, the induced voltages at high frequencies yield Eddy currents large enough to attenuate the applied B1 field with increasing depth in the imaged object. As a consequence, by using phantoms which have a dielectric constant similar to the dielectric constant of human tissue, the homogeneity and uniformity of an acquired MR image of the phantom at higher magnetic field strength greater than 1.5 T is degraded. At lower fields, however toxic compounds (which need a proper handling and logistic) are sometimes used in order to achieve loading properties comparable with body.
Thus, as phantom material either a material with a dielectric constant and electrical conductivity which allows avoiding dielectric resonances needs to be used for testing purposes, which however has the disadvantage that such a material is not a representative of human tissue. Or a phantom material with a dielectric constant and electrical conductivity similar to the dielectric constant of human tissue needs to be used which however strongly distorts the B1 field.
In order to find a compromise between these two approaches, for example WO 2008/014445 suggests using a micelle solution to reduce dielectric resonance effects in MRI phantoms.